1. Field of the Invention
The present invention relates to fluid handling apparatus, and more particularly to marine tanker loading arms with systems for controlling the movements of the arm sections.
2. Description of the Prior Art
Various types of fluid transferring devices are used in conveying fluids between a dock, buoy, or floating barge and tanker or other marine transport vessel. Some of these devices are large articulated pipe structures referred to as marine loading arms, such an arm generally comprising an upstanding riser on which are mounted two or more arm members that are pivotally interconnected by swivel joints in a manner to allow the arm to accommodate itself to movements of the buoy, barge and/or tanker while fluid transfer is in progress. Some examples of these prior art fluid transferring devices are disclosed in the U.S. Pat. Nos. 2,980,150 and 3,382,892 to Bily, 3,581,769 to Haley, and 3,889,728 to Riche.
It is common practice to install weights to counterbalance a loading arm about the horizontal pivot axis at the riser so that it may be more easily maneuvered. To achieve a constant balanced condition, in some marine loading arms the weight of the counterbalance is varied by adding or subtracting water or other fluid from a container positioned on the counterbalance or its support. Since the weight per unit volume of the liquid is relatively low, this type of counterbalance system has the disadvantage of requiring a large container for the liquid, and also a pumping system to move the liquid into the container. Other counterbalancing systems employ movable counterweights that can be repositioned on their supports to compensate for changes in the weight of the arm members that occur as these members are filled with or emptied of fluid during start up and termination of the fluid transfer operation. One difficulty with this latter type of system is that it is not sufficiently responsive to constantly maintain the arm in a balanced condition during these changes in weight.
Another difficulty with some of the prior art counterbalancing systems is that the counterweights are positioned at a considerable distance above the deck of the ship or the dock on which the loading arm is mounted. This causes the center of gravity to be relatively high, thereby producing undue stress on the riser which supports both the arm members and the counterweights. Counterweights also present a safety hazard to people who may be working beneath them, for these weights usually weigh several thousand pounds. Thus, it is desirable that all counterweights be mounted as near as possible to the level of the deck or dock. However, a counterweight mounted near the deck or dock may pivot downward beyond its normally lowest position and damage or interfere with valves, pressure gauges, piping, and other objects mounted on the dock, when the outboard end of the arms is allowed to rise above the highest level for which it was designed. This problem can occur during normal manipulations of the arm as well as when the arm is quickly disconnected from the tanker manifold, such as when an emergency situation develops.